Studies continue to center on the relationship between the miracidium of Schistosoma mansoni and its intermediate host-snail, Biomphalaria glabrata. We are continuing current efforts to isolate and identify the snail-produced substance that alters the swimming behavior and host-finding capacity of the miracidium. Other studies on the miracidium indicate (1) that only about one-third of those freshly hatched from infected mouse tissue are capable of penetrating susceptible snails, and (2) that miracidia of a Puerto Rican strain are capable of infecting neonatal snails (less than 1mm diam) of Brazilian origin which cannot be infected when the snails reach 4 to 8 mm diam. Bibliographic references: Chernin, E. and Antolics, V.M. 1975. Neonatal susceptibility of Brazilian Biomphalaria to Puerto Rican Schistosoma mansoni. J. Parasit. 61:377-378. Chernin, E. and Antolics, V.M. 1975. Penetrative capacity of Schistosoma mansoni miracidia. J. Parasit. 61:560-561.